Green’s Dictionary of Slang

1. a martini and pernod cocktail.

1968

K. Brasselle Cannibals 157: I’ll make us both another shooter.

2. (US) a measure of spirits, esp. whisky; tequila.

1990		S. Morgan Homeboy 97: Geez, he could use a shooter of whiskey.
1994		J. Wambaugh Finnegan’s Week 210: The ox just wanted to drink tequila shooters [...] He signaled for another shooter.